The present invention relates to a branching filter useful in a compact communication device such as a portable telephone set.
Compact, lightweight portable telephone sets and other portable communication equipment have been undergoing intensive development in recent years. Small, high-performance components are required. The branching filter, which is used as a radio-frequency (RF) filter in such equipment, is one of these components. Incidentally, a branching filter is also referred to as an antenna duplexer.
FIG. 1 is a schematic diagram of a first conventional branching filter, comprising an antenna port 1, inductors 2, 3, 4, a transmitting (TX) filter 5, a receiving (RX) filter 6, a transmitting port 7, and a receiving port 8. The transmitting filter 5 and receiving filter 6 employ dielectric resonators. FIG. 2 is a perspective view of this branching filter, showing the substrate 9 on which the transmitting filter 5 and receiving filter 6 are mounted. FIG. 3 is a perspective view of the underside of the substrate 9, showing the metalized pattern 9a from which the inductors 2, 3, 4 are formed, and the insulating pattern 9b in which the surface of the substrate 9 is exposed.
This first conventional branching filter has good isolation characteristics, because the transmitting filter 5 and receiving filter 6 are mounted separately, but it requires a comparatively large substrate 9 with special patterning to form the inductors 2, 3, 4. There are also problems of mutual interference between interconnecting lines and inadequate separation between the transmitting frequency band and receiving frequency band. A compact, high-performance filter is difficult to achieve with this design.
Surface-acoustic-wave (SAW) filters of the resonator type provide a way to achieve a more compact branching-filter configuration. FIG. 4 shows a second conventional branching filter housed in a single package 10. The interior 11A of the package includes a ground plane 11B, an impedance-matching element 12, a phase-matching element 13, and a trap circuit 14, as well as a transmitting SAW-filter element 15 and a receiving SAW-filter element 16, as described in Japanese Unexamined Patent Application 06097761.
Placing all of the elements of the second conventional branching filter in a single package, however, leads to the problems noted above: mutual interference between interconnecting lines, and inadequate separation between the transmitting and receiving frequency bands.
In the first conventional branching filter, the transmitting filter 5 and receiving filter 6 are sealed in separate packages, but as noted above, the inductors 2, 3, 4 are formed on the substrate 9, increasing the space occupied by the substrate 9 and precluding the design of a very compact branching filter. Furthermore, the basic filter design in FIG. 1 leads to the above-mentioned problems of mutual interference between interconnecting lines and inadequate separation between the transmitting and receiving frequency bands. The second conventional branching filter in FIG. 4, which employs SAW resonators, also exhibits the same problems of mutual interference between interconnecting lines and inadequate separation between the transmitting and receiving frequency bands.
It is accordingly an object of the present invention to provide a compact, high-performance branching filter.
According to the present invention, a branching filter comprises a first package and a second package. The first package encloses a transmitting filter of the SAW-resonator type. The second package encloses a receiving filter of the SAW-resonator type. The transmitting filter and receiving filter have different passbands. Other necessary elements, such as striplines having a certain length, are incorporated into the first and second packages.